Strong friendships can help voting spread like a contagion.

Despite the importance of Congressional representation, barely a third of eligible US voters cast a ballot in the most recent midterm elections. Various get-out-the-vote campaigns have been tried, and many of these have been shown to have a positive effect on voting, but most of them are too focused to reach a large portion of the population. Now, some researchers have tested a method with a good deal more reach: an appeal to potential voters via Facebook. Thanks to the heavy use of that social networking service, the study had the largest experimental population I've ever seen, at 61 million.

On the day of the midterm election, every potential voter who logged in got an ad that encouraged them to vote. And, to a small extent, it worked—voting among those users edged up ever so slightly, and was enhanced when their close friends voted. Given the size of the appeal, this small boost translated to hundreds of thousands of additional voters.

The procedure for the experiment was remarkably simple. Anyone in the US who was over 18 and logged in to Facebook the day of the 2010 election was enrolled in the study. The authors note that this probably makes the numbers they obtained an underestimate of the procedure's effects, since some of these people will have logged in after polls closed.

The subjects were divided into three groups. One control population group, with a bit over 600,000 subjects, didn't receive any special message. Another population of 600,000 was shown a message encouraging them to vote, a link to a site that would help them find the local polling place (clicks were tracked), and a button to click that registered that they had voted, along with a running count of users who had previously clicked this button (clicks were also tracked). A third group—and this was the big one, with over 60 million people—got the same encouragement and buttons, but the buttons were accompanied by a set of profile pictures of friends who had also voted.

The setup tracked the number of people who looked up polling information, along with the number that claimed they had voted. To verify the accuracy of this self-reporting, the researchers compared it to the public voting records of 6.3 million people.

The intervention had a small effect but, with a study population this large, it was actually possible to measure even a tiny influence. Those who received the message to vote accompanied by social cues were 0.39 percent more likely to vote than the group that received no message. But they were also that much more likely to have actually voted than the group that had been urged to vote without any accompanying social cues. In fact, the authors found that there was little difference between not receiving a pro-vote message and getting it without any reminders that their friends were voting.

That may seem like a minimal impact but, nationwide, this would be expected to boost the voter population by 340,000 people. Presumably, some of them voted in highly competitive races.

As you might expect, however, people cared more about appearances than actually voting. When it came to self-reported voting (through the button displayed in the tests), the socially enhanced message got a full two percent more people to say they had voted.

To track how social cues worked, the authors estimated the strength of a friendship by seeing how often pairs of users had interacted over the preceding month. In the population for which the authors had voting data, weak friendships had no effect, but voters gave closer friends a significant inducement to vote, and even weak friendship boosted self-reported voting. So, the authors conclude, "Ordinary Facebook friends may affect online expressive behavior, but they do not seem to affect private or real-world political behaviors. In contrast, close friends seem to have influenced all three."

This is a sign of what's referred to as "social contagion," where a behavior spreads through groups via strong social links. In the case of voting, the social aspect ended up being stronger than the direct appeal of the image that suggested people should vote. The authors estimate the direct impact of the ad as pushing 60,000 people to vote, while the added effect of social contagion accounted for 280,000 additional voters.

The authors note that past studies of online inducement had suggested they had little effect on voting behavior. However, generating social contagion in a familiar context, like that of Facebook, may be easier than other approaches—after all, everybody hates chain e-mails, but forwards on all sorts of crap via Facebook. And given that the effect was small, it was probably pretty easy to miss it in studies without a 60 million-strong experimental population.